1. Field of the Invention
The present invention relates to a PLC-type delay demodulation circuit including a planar lightwave circuit that is formed on one PLC chip and demodulates a DQPSK modulated optical signal.
2. Description of the Related Art
In a 40 Gbps DQPSK communication system, as a method of configuring a delay circuit that demodulates a DQPSK (Differential Quadrature Phase Shift Keying)-modulated signal (optical signal) in a PLC, a method has been proposed which configures the delay circuit including an optical splitter and two MZIs (for example, see Hashimoto, Toshikazu, et al., “Compact DQPSK Demodulator with Interwoven Double Mach-Zehnder Interferometer using Planar Lightwave Circuit,” ECOC 2008 Proceeding, Mo.3.C.2). In the device, it is necessary to reduce the size of a module, power consumption, and polarization dependence and obtain uniform MZI characteristics.
Of these, particularly for the reduction of polarization dependence, regarding a delay circuit (optical signal processor) including a single MZI, it has been proposed that two arm waveguides that form the MZI are brought close to each other in a wave plate insertion portion so as to reduce the effect of the positional dependence of the polarization conversion efficiency of the wave plate, thereby reducing polarization dependence (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2007-232944).
Similarly, the deterioration of polarization dependence due to the positional dependence of the polarization conversion efficiency of the wave plate also occurs in a delay circuit having two MZIs for DQPSK signal demodulation. Thus, for example, in the delay circuit for DQPSK described in Japanese Patent Application Laid-Open (JP-A) No. 2009-244483, as shown in FIG. 14, the following attempt has been made. In the portion where a wave plate 47 is inserted, first arm waveguides 8 and 12 and second arm waveguides 9 and 13 of two MZIs 4 and 5 are brought close to each other, respectively, so as to reduce the effect of the positional dependence of the polarization conversion efficiency of the wave plate 47 between the two arm waveguides for each of the MZIs 4 and 5, thereby reducing polarization dependence.
However, in the case where one wave plate 47 is inserted for the arm waveguides of the MZI 4 and MZI 5 and used as shown in FIG. 14, in addition to the positional dependence of the polarization conversion efficiency of the wave plate at the position where the wave plate intersects the two arm waveguides of each MZI, a problem may arise in the positional dependence of the polarization conversion efficiency of the wave plate 47 between the positions of the arm waveguides 8 and 9 of the MZI 4 and the positions of the arm waveguides 12 and 13 of the MZI 5. That is, for example, because of the positional dependence of the polarization conversion efficiency of the wave plate 47, although a portion with excellent polarization conversion efficiency can used for one MZI 4, a portion with poor polarization conversion efficiency is assigned to the other MZI 5, resulting in a problem in that it may be impossible to obtain excellent characteristics in both the MZIs 4 and 5 at the same time.
Further, a wave plate that is sized to intersect each of the arm waveguides of both the MZIs 4 and 5 is required, resulting in the problem of increased cost.
The problem of the positional dependence of the polarization conversion efficiency of the wave plate can be solved by using different wave plates for respective MZIs. However, in such a case, there are problems such as the cost for the wave plates or their insertion, an increase in size due to the space to be ensured for the insertion of the two wave plates, etc.
In light of these problems and requirements, the present invention proposes a novel layout for a PLC-type delay demodulation circuit. An object of the present invention is to provide a PLC-type delay demodulation circuit that is less susceptible to the positional dependence of the polarization conversion efficiency of a wave plate, facilitates the achievement of excellent characteristics in both MZIs at the same time, and enables cost reduction.